A currently popular heat-transfer fluid consists of a eutectic mixture of diphenyl ether and biphenyl. The diphenyl ether/biphenyl eutectic mixture has good thermal stability up to 750.degree. F. and, therefore, has a maximum recommended use temperature of 750.degree. F. Unfortunately, the fluid freezes at about 54.degree. F. This high freezing point requires users to heat-trace their systems in order to start up or shut down anywhere the temperature may drop below the 54.degree. F. The need to heat-trace results in increased capital, maintenance, and operating expenses. There exists other heat-transfer fluids which have lower freezing points (also referred to as "minimum use temperatures") but they do not have high maximum recommended use temperatures. Therefore, there is a need for an improved heat-transfer fluid having a low freezing point and a relatively high maximum recommended use temperature.
Additionally, there exist needs for improved lubricants and hydraulic fluids. The demands placed on these materials continue to undergo significant changes. Lubricants and hydraulic fluids for future automotive and aeronautic applications will have requirements different from those currently in use. For example, it is anticipated that engines will operate at temperatures exceeding 480.degree. F. and will be constructed using materials different from those currently in use. Thus, what are needed are new compositions useful as lubricants or hydraulic fluids that are stable at the high use temperature while possessing the other properties required of these materials.
Materials such as heat-transfer fluids, lubricants, and hydraulic fluids may be considered "functional fluids" because of their usefulness in mechanical operations. Examples of previous attempts of preparing such functional fluids are described in the following patents.
U.S. Pat. No. 4,622,160 issued to Buske et al. on Nov. 11, 1986, discloses heat transfer fluids which are made by mixing methyl and/or ethyl biphenyls with diethylbenzenes. The heat transfer fluids may contain one or more extenders, which extenders include diphenyl oxide and/or biphenyl.
U.S. Pat. No. 4,054,533 issued to Watson on Oct. 18, 1977, discloses heat transfer fluids consisting essentially of about, by weight: (a) 20 to about 40 percent of diphenyl ether, (b) 40 to about 60 percent of a 2-biphenylyl phenyl ether and 4-biphenylyl phenyl ether mixture, and (c) 12 to about 25 percent of a polyphenylphenol mixture. Minor amounts, typically less than about 7 weight percent, of various impurities may be present, which impurities include phenylnaphthalene, methylphenylnaphthalene and the like. U.S. Pat. No. 3,966,626 issued to Jackson et al. on Jun. 29, 1976, discloses heat transfer agents containing at least about 20 volume percent diphenyl oxide and at least 20 volume percent biphenylyl phenyl ether, polyphenyl ether or mixtures thereof.
U.S. Pat. No.3,931,028 issued to Jackson et al. on Jan. 6, 1976, discloses that a well-known heat transfer fluid consisting essentially of the eutectic mixture of diphenyl oxide and biphenyl is improved by the addition of monomethyl or monoethylbiphenyl.
U.S. Pat. No. 3,907,696 issued to Jackson et al. on Sep. 23, 1975, discloses heat transfer agents containing at least three components consisting of by volume 5 to 90 percent diphenyl oxide, 5 to 50 percent biphenyl and 5 to 90 percent polyphenyl ether having 3 or 4 aromatic nuclei, alkylated biphenyl or diphenyl oxide having 1 to 4 methyl or ethyl substituents, ethylbenzene oil and mixtures thereof.
U.S. Pat. No. 3,888,777 issued to Jackson et al. on Jun. 10, 1975, discloses a three-component heat transfer agent containing 5 to 90 percent diphenyl oxide, 5 to 50 percent biphenyl and 5 to 90 percent biphenylyl phenyl ether.
U.S. Pat. No. 3,243,380 issued to Conn on Mar. 29, 1966, discloses a method of decontaminating an aromatic hydrocarbon liquid moderator-coolant. For evaluation purposes, liquid triphenyl products were used with determined amounts of dibenzothiophene admixed.
U.S. Pat. No. 3,113,090 issued to Ort et al. on Dec. 3, 1963, discloses a composition for cooling neutronic reactor comprising polyphenyl and anthracene. Polyphenyl is defined in the patent to include biphenyl, terphenyls, alkylation products thereof and mixtures thereof.
U.S. Pat. No. 2,883,331 issued to Bolt et al. on Apr. 21, 1959, discloses inhibited reactor coolants comprising certain polynuclear aromatic hydrocarbons, certain aryl selenium and sulfur compounds. The certain aryl selenium and sulfur compounds contain at least two phenyl rings and at least one sulfur or selenium atom or both, the compounds being substantially free of elements other than C, H, S and Se. Suitable polynuclear aromatic hydrocarbons include biphenyl and naphthalene.
U.S. Pat. No. 2,033,702 issued to Grebe et al. on Mar. 10, 1936, discloses a heat transfer agent comprising a mixture of polynuclear aromatic hydrocarbons higher than diphenyl which is formed through the condensation between benzene and/or diphenyl in the manufacture of diphenyl by pyrolysis of benzene. The material is obtained as a residue after distilling off unreacted benzene and diphenyl product from the reacted mixture. The patent describes that the exact composition of the residual mixture is not known, but it contains a considerable proportion of triphenyl, tetraphenyls, etc.
U.S. Pat. No. 1,972,847 issued to Levine et al. on Sep. 4, 1934, discloses a stable heat transfer medium comprising diphenyl oxide, naphthalene and diphenyl.
U.S. Pat. No. 1,882,809 issued to Grebe on Oct. 18, 1932, discloses a heat transfer fluid having a hiqh boiling point and a low freezing point comprising diphenyloxide and diphenyl. Grebe mentions that after using the fluid continuously for some time, a gradual decomposition will occur wherein products of higher and lower boiling points will be formed, and that the formation of such decomposition products still further lowers the final freezing point of the mixture.
U.S. Pat. No. 1,874,258 issued to Dow on Aug. 30, 1932, discloses a heating fluid which consists of diphenyl oxide, diphenylene oxide (also known as dibenzofuran), and, optionally, diphenyl or naphthalene.
French Patent No. 960,087 issued to the Standard-Thomson Corporation of the United States on Oct. 17, 1949, discloses liquids for filling thermostatic devices which include a mixture which is essentially the eutectic mixture of diphenyl ether with two or three of the following compounds: diphenyl, metaterphenyl, orthoterphenyl, and benzophenone.
It is, therefore, an object of the present invention to provide a functional fluid composition (a) which is useful as either a heat-transfer fluid, a lubricant, or a hydraulic fluid, and the like, (b) which has a low freezing point and a high maximum recommended use temperature, (c) which contains components which are mutually soluble and capable of preparing homogeneous mixtures, (d) wherein the components have relatively close boiling points for easier recovery and purification by distillation, and (d) which are made from readily available materials.